Superhard constructions and methods of making same

ABSTRACT

A superhard polycrystalline construction comprises a body of polycrystalline superhard material comprising a superhard phase, and a second phase dispersed in the superhard phase, the superhard phase comprising a plurality of inter-bonded superhard grains. The second phase comprises particles or grains that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % or wt % of the body of polycrystalline superhard material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application based on U.S. Ser. No. 15/031,058, filed on Apr. 21, 2016, which is a 371 National Stage of International Application No. PCT/EP2014/072682, filed Oct. 22, 2014, which claims priority to GB1318640.8, filed Oct. 22, 2013.

FIELD

This disclosure relates to superhard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline superhard materials, such as polycrystalline diamond (PCD) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of super hard tool inserts may be limited by fracture of the super hard material, including by spalling and chipping, or by wear of the tool insert.

Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a super hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the super hard material layer is typically polycrystalline diamond (PCD), or a thermally stable product TSP material such as thermally stable polycrystalline diamond.

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material or ultra hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering.

Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with a superhard material layer such as PCD, diamond particles or grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains occurs, forming a polycrystalline superhard diamond layer.

In some instances, the substrate may be fully cured prior to attachment to the superhard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the superhard material layer.

Ever increasing drives for improved productivity in the earth boring field place ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements or tool inserts comprising PCD material are widely used in drill bits for boring into the earth in the oil and gas drilling industry. Rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite.

The most wear resistant grades of PCD usually suffer from a catastrophic fracture of the cutter before it has worn out. During the use of these cutters, cracks grow until they reach a critical length as which catastrophic failure occurs, namely, when a large portion of the PCD breaks away in a brittle manner. These long, fast growing cracks encountered during use of conventionally sintered PCD, result in short tool life.

Furthermore, despite their high strength, polycrystalline diamond (PCD) materials are usually susceptible to impact fracture due to their low fracture toughness. Improving fracture toughness without adversely affecting the material's high strength and abrasion resistance is a challenging task.

There is therefore a need for a polycrystalline super hard composite such as a PCD composite that has good or improved abrasion, fracture and impact resistance and a method of forming such composites.

SUMMARY

Viewed from a first aspect there is provided a superhard polycrystalline construction comprising:

a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising:

a superhard phase, and a second phase dispersed in the superhard phase, the superhard phase comprising a plurality of inter-bonded superhard grains;

wherein the second phase comprises particles or grains that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % of the body of polycrystalline superhard material.

Viewed from a second aspect there is provided a method of forming a superhard polycrystalline construction, comprising:

-   -   providing a mass of particles or grains of superhard material;

providing a mass of second phase grains or particles comprising particles or grains of a material that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % of the body of polycrystalline superhard material;

combining the mass of superhard material and the mass of second phase grains to form a pre-sinter assembly; and

treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 5 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the second phase being dispersed in the polycrystalline material and forming between around 1 to around 30 vol % of the body of polycrystalline superhard material, any residual catalyst/solvent at least partially filling a plurality of the interstitial regions.

Viewed from a further aspect there is provided a tool comprising the superhard polycrystalline construction defined above, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

The tool may comprise, for example, a drill bit for earth boring or rock drilling, a rotary fixed-cutter bit for use in the oil and gas drilling industry, or a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

Viewed from another aspect there is provided a drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of an example PCD cutter element for a drill bit for boring into the earth;

FIG. 2 is a schematic cross-section of an example portion of a PCD micro-structure with a second dispersed non-reactive phase in the material;

FIG. 3 is a plot showing the results of a vertical borer test comparing a conventional PCD material, an embodiment of PCD material comprising 30 vol % (around 30 wt %) second phase dispersed therein, and an embodiment of PCD material comprising 10 vol % (around 10 wt %) second phase dispersed therein and prepared according to the described method;

FIG. 4 is a plot showing the results of a vertical borer test comparing a conventional PCD material, an embodiment of PCD material comprising 7.5 wt % second phase dispersed therein, and an embodiment of PCD material comprising 20 wt % second phase dispersed therein and prepared according to the described method, for the conditions where the average grain size of the second phase material is substantially equal to that of the diamond and where the average grain size of the second phase material is larger than that of the diamond grains;

FIG. 5 is a plot showing the results of a vertical borer test comparing a conventional PCD material, an embodiment of PCD material comprising 7.5 wt % second phase dispersed therein, and an embodiment of PCD material comprising 20 wt % second phase dispersed therein and prepared according to the described method, for the conditions where the average grain size of the second phase material is less than that of the diamond, where the average grain size of the second phase material is substantially equal to that of the diamond, where the average grain size of the second phase material is approximately double that of the diamond, and where the average grain size of the second phase material is larger than double that of the diamond grains; and

FIG. 6 is a plot showing the results of a vertical borer test comparing a conventional PCD material, an embodiment of PCD material comprising 7.5 wt % second phase dispersed therein, and an embodiment of PCD material comprising 20 wt % second phase dispersed therein and prepared according to the described method, for the conditions where the average grain size of the second phase material is less than that of the diamond grains, where the average grain size of the second phase material is approximately equal to that of the diamond grains and where the average grain size of the second phase material is larger than that of the diamond grains.

The same references refer to the same general features in all the drawings.

DESCRIPTION

As used herein, a “superhard material” is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of superhard materials.

As used herein, a “superhard construction” means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In one example of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, “interstices” or “interstitial regions” are regions between the diamond grains of PCD material. In examples of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A “catalyst material” for a superhard material is capable of promoting the growth or sintering of the superhard material.

The term “substrate” as used herein means any substrate over which the superhard material layer is formed. For example, a “substrate” as used herein may be a transition layer formed over another substrate.

As used herein, the term “integrally formed” regions or parts are produced contiguous with each other and are not separated by a different kind of material.

In an example as shown in FIG. 1, a cutting element 1 includes a substrate 10 with a layer of superhard material 12 formed on the substrate 10. The substrate 10 may be formed of a hard material such as cemented tungsten carbide. The superhard material 12 may be, for example, polycrystalline diamond (PCD), or a thermally stable product such as thermally stable PCD (TSP). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown) and may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

The exposed top surface of the superhard material opposite the substrate forms the cutting face 14, which is the surface which, along with its edge 16, performs the cutting in use.

At one end of the substrate 10 is an interface surface 18 that forms an interface with the superhard material layer 12 which is attached thereto at this interface surface. As shown in the embodiment of FIG. 1, the substrate 10 is generally cylindrical and has a peripheral surface 20 and a peripheral top edge 22.

As used herein, a PCD grade is a PCD material characterised in terms of the volume content and size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. A grade of PCD material may be made by a process including providing an aggregate mass of diamond grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for diamond to a pressure and temperature at which diamond is more thermodynamically stable than graphite and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a PCD structure. The aggregate mass may comprise loose diamond grains or diamond grains held together by a binder material and said diamond grains may be natural or synthesised diamond grains.

Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K₁C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

All of the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The PCD structure 12 may comprise one or more PCD grades.

FIG. 2 is a cross-section through an example of PCD material forming the superhard layer 12 of FIG. 1 showing, schematically, the PCD microstructure. A second phase comprising particles 30 or agglomerates formed of particles or grains that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % (or 1 to 30 weight %) of the body of polycrystalline superhard material, is dispersed in the diamond phase matrix 32. The diamond phase matrix 32 here refers to conventional PCD formed of diamond grains and a catalyst binder phase dispersed therein. The second phase 30 may comprise, for example any one or more of an oxide material, a carbide material, alumina, zirconia, yttria, silica, tantalum oxide, cBN, PCBN, hexagonal boron nitride, tungsten carbide, hafnium carbide, zirconium carbide, silicon carbide, or any combination thereof. The grain size of the dispersed second phase particles 30 may, in some examples, be the same size as or between around 1.5 to around 3 times greater than the average size of the diamond particles or grains, for example, around twice the size. The second phase particles 30 may, for example, be admixed with diamond powder and then the composite is sintered in the traditional way with, for example, cobalt infiltration at HPHT to provide the catalyst binder to form the intergranular bonds between the diamond grains.

In some examples, the second phase particles 30 comprise between around 1 to around 30 volume % (or around 1 to around 30 wt %) of the sintered PCD material. In other examples, the second phase particles 30 comprise between around 5 to around 15 volume % (or around 5 to around 50 wt %) of the sintered PCD material, or even, in some cases, between around 10 to between around 30 volume % (or around 10 to around 30 wt %) of the sintered PCD material.

In some examples, the average size of the grains or particles forming the second phase 30 is between around 10 microns to around 350 microns and in some examples below around 125 microns.

These disperse second phase particles 30 may be localized inside the binder pools or in between diamond grains, depending on the sizes.

The grains of superhard material may be, for example, diamond grains or particles. In the starting mixture prior to sintering they may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains. In some examples, the coarse fraction may have, for example, an average particle/grain size ranging from about 10 to 60 microns. By “average particle or grain size” it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the “average”. The average particle/grain size of the fine fraction is less than the size of the coarse fraction. For example, the fine fraction may have an average grain size of between around 1/10 to 6/10 of the size of the coarse fraction, and may, in some examples, range for example between about 0.1 to 20 microns.

In some examples, the weight ratio of the coarse diamond fraction to the fine diamond fraction may range from about 50% to about 97% coarse diamond and the weight ratio of the fine diamond fraction may be from about 3% to about 50%. In other examples, the weight ratio of the coarse fraction to the fine fraction may range from about 70:30 to about 90:10.

In further examples, the weight ratio of the coarse fraction to the fine fraction may range for example from about 60:40 to about 80:20.

In some examples, the particle size distributions of the coarse and fine fractions do not overlap and in some examples the different size components of the compact are separated by an order of magnitude between the separate size fractions making up the multimodal distribution.

Some examples consist of a wide bi-modal size distribution between the coarse and fine fractions of superhard material, but some examples may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude, for example, a blend of particle sizes whose average particle size is 20 microns, 2 microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In examples where the superhard material is polycrystalline diamond material, the diamond grains used to form the polycrystalline diamond material may be natural or synthetic.

In some examples, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some examples, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some examples, the metal carbide is tungsten carbide.

The cutter of FIG. 1 having the microstructure of FIG. 2 may be fabricated, for example, as follows.

As used herein, a “green body” is a body comprising grains to be sintered and a means of holding the grains together, such as a binder, for example an organic binder, together with the additional second phase 30.

Examples of superhard constructions may be made by a method of preparing a green body comprising grains or particles of superhard material, second phase and a binder, such as an organic binder. The green body may also comprise catalyst material for promoting the sintering of the superhard grains. The green body may be made by combining the grains or particles with the binder/catalyst and forming them into a body having substantially the same general shape as that of the intended sintered body, and drying the binder. At least some of the binder material may be removed by, for example, burning it off. The green body may be formed by a method including a compaction process, an injection process or other methods such as molding, extrusion, deposition modelling methods.

A green body for the superhard construction may be placed onto a substrate, such as a pre-formed cemented carbide substrate to form a pre-sinter assembly, which may be encapsulated in a capsule for an ultra-high pressure furnace, as is known in the art. The substrate may provide a source of catalyst material for promoting the sintering of the superhard grains. In some examples, the superhard grains may be diamond grains and the substrate may be cobalt-cemented tungsten carbide, the cobalt in the substrate being a source of catalyst for sintering the diamond grains. The pre-sinter assembly may comprise an additional source of catalyst material.

In one version, the method may include loading the capsule comprising a pre-sinter assembly into a press and subjecting the green body to an ultra-high pressure and a temperature at which the superhard material is thermodynamically stable to sinter the superhard grains. In some examples, the green body may comprise diamond grains and the pressure to which the assembly is subjected is at least about 5 GPa and the temperature is at least about 1,300 degrees centigrade.

A version of the method may include making a diamond composite structure by means of a method disclosed, for example, in PCT application publication number WO2009/128034 with the additional step of admixing with the diamond grains, prior to sintering, the grains/particles of non-reactive phase. A powder blend comprising diamond particles, second phase particles and a metal binder material, such as cobalt may be prepared by combining these particles and blending them together. An effective powder preparation technology may be used to blend the powders, such as wet or dry multi-directional mixing, planetary ball milling and high shear mixing with a homogenizer. In one embodiment, the mean size of the diamond particles may be at least about 50 microns and they may be combined with other particles by mixing the powders or, in some cases, stirring the powders together by hand. In one version of the method, precursor materials suitable for subsequent conversion into binder material may be included in the powder blend, and in one version of the method, metal binder material may be introduced in a form suitable for infiltration into a green body. The powder blend may be deposited in a die or mold and compacted to form a green body, for example by uni-axial compaction or other compaction method, such as cold isostatic pressing (CIP). The green body may be subjected to a sintering process known in the art to form a sintered article. In one version, the method may include loading the capsule comprising a pre-sinter assembly into a press and subjecting the green body to an ultra-high pressure and a temperature at which the superhard material is thermodynamically stable to sinter the superhard grains.

After sintering, the polycrystalline super hard constructions may be ground to size and may include, if desired, a 45° chamfer of approximately 0.4 mm height on the body of polycrystalline super hard material so produced.

The sintered article may be subjected to a subsequent treatment at a pressure and temperature at which diamond is thermally stable to convert some or all of the non-diamond carbon back into diamond and produce a diamond composite structure. An ultra-high pressure furnace well known in the art of diamond synthesis may be used and the pressure may be at least about 5.5 GPa and the temperature may be at least about 1,250 degrees centigrade for the second sintering process.

A further example of a superhard construction may be made by a method including providing a PCD structure and a precursor structure for a diamond composite structure, forming each structure into the respective complementary shapes, assembling the PCD structure and the diamond composite structure onto a cemented carbide substrate to form an unjoined assembly, and subjecting the unjoined assembly to a pressure of at least about 5.5 GPa and a temperature of at least about 1,250 degrees centigrade to form a PCD construction. The precursor structure may comprise carbide particles and diamond or non-diamond carbon material, such as graphite, second phase particles, and a binder material comprising a metal, such as cobalt. The precursor structure may be a green body formed by compacting a powder blend comprising particles of diamond or non-diamond carbon and particles of carbide material and compacting the powder blend.

In some examples, both the bodies of, for example, diamond and carbide material plus the second phase and sintering aid/binder/catalyst are applied as powders and sintered simultaneously in a single UHP/HT process. The mixture of diamond grains, second phase particles and mass of carbide are placed in an HP/HT reaction cell assembly and subjected to HP/HT processing. The HP/HT processing conditions selected are sufficient to effect intercrystalline bonding between adjacent grains of abrasive particles and, optionally, the joining of sintered particles to the cemented metal carbide support. In one example, the processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least about 1200 degrees C. and an ultra-high pressure of greater than about 5 GPa.

In another example, the substrate may be pre-sintered in a separate process before being bonded together in the HP/HT press during sintering of the ultrahard polycrystalline material.

In a further example, both the substrate and a body of polycrystalline superhard material are pre-formed. For example, the bimodal feed of ultrahard grains/particles with second phase particles and optional carbonate binder-catalyst also in powdered form are mixed together, and the mixture is packed into an appropriately shaped canister and is then subjected to extremely high pressure and temperature in a press. Typically, the pressure is at least 5 GPa and the temperature is at least around 1200 degrees C. The preformed body of polycrystalline superhard material is then placed in the appropriate position on the upper surface of the preform carbide substrate (incorporating a binder catalyst), and the assembly is located in a suitably shaped canister. The assembly is then subjected to high temperature and pressure in a press, the order of temperature and pressure being again, at least around 1200 degrees C. and 5 GPa respectively. During this process the solvent/catalyst migrates from the substrate into the body of superhard material and acts as a binder-catalyst to effect intergrowth in the layer and also serves to bond the layer of polycrystalline superhard material to the substrate. The sintering process also serves to bond the body of superhard polycrystalline material to the substrate.

In examples where the cemented carbide substrate does not contain sufficient solvent/catalyst for diamond, and where the PCD structure is integrally formed onto the substrate during sintering at an ultra-high pressure, solvent/catalyst material may be included or introduced into the aggregated mass of diamond grains from a source of the material other than the cemented carbide substrate. The solvent/catalyst material may comprise cobalt that infiltrates from the substrate in to the aggregated mass of diamond grains just prior to and during the sintering step at an ultra-high pressure. However, in embodiments where the content of cobalt or other solvent/catalyst material in the substrate is low, particularly when it is less than about 11 weight percent of the cemented carbide material, then an alternative source may need to be provided in order to ensure good sintering of the aggregated mass to form PCD.

Solvent/catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent/catalyst material in powder form with the diamond grains, depositing solvent/catalyst material onto surfaces of the diamond grains, or infiltrating solvent/catalyst material into the aggregated mass from a source of the material other than the substrate, either prior to the sintering step or as part of the sintering step. Methods of depositing solvent/catalyst for diamond, such as cobalt, onto surfaces of diamond grains are well known in the art, and include chemical vapour deposition (CVD), physical vapour deposition (PVD), sputter coating, electrochemical methods, electroless coating methods and atomic layer deposition (ALD). It will be appreciated that the advantages and disadvantages of each depend on the nature of the sintering aid material and coating structure to be deposited, and on characteristics of the grain.

Similarly, the second phase particles may be introduced by various means, for example, the diamond grains or particles could be coated with a non-reacting material prior to sintering.

In one example, the binder/catalyst such as cobalt may be deposited onto surfaces of the diamond grains by first depositing a pre-cursor material and then converting the precursor material to a material that comprises elemental metallic cobalt. For example, in the first step cobalt carbonate may be deposited on the diamond grain surfaces using the following reaction:

Co(NO₃)₂+Na₂CO₃→CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or other solvent/catalyst for diamond may be achieved by means of a method described in PCT patent publication number WO/2006/032982. The cobalt carbonate may then be converted into cobalt and water, for example, by means of pyrolysis reactions such as the following:

CoCO₃→COO+CO₂

CoO+H₂→Co+H₂O

In another example, cobalt powder or precursor to cobalt, such as cobalt carbonate, may be blended with the diamond grains. Where a precursor to a solvent/catalyst such as cobalt is used, it may be necessary to heat treat the material in order to effect a reaction to produce the solvent/catalyst material in elemental form before sintering the aggregated mass.

In some examples, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate. The binder material may comprise between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprises Co.

Versions are described in more detail below with reference to the following examples which are provided herein by way of illustration only and are not intended to be limiting.

Example 1

Powders of the second phase materials are mixed in an organic binder such as polyvinyl alcohol (PVA) or Poly(methyl methacrylate) (PMMA) and de-ionised water to form the first slurry. The slurry is then freeze granulated in liquid nitrogen (liquid nitrogen is used to freeze the slurry depending on the size of the nozzle which determines the size of the resulting granules).

In a first example, cBN powder (grade 9) was mixed with PVA organic binder and de-ionised water was used to prepare granules smaller than 350 microns. The water was removed from the granules during freeze drying. The cBN particles are bonded together by the organic binder (PVA). The dried powder (granules) was sieved to recover granules of the desired size, in this example, granules smaller than around 125 micron were recovered.

A second slurry of diamond powder with a mixture of different grain sizes, ranging between around 1 micron to around 40 microns was prepared using a solvent such as ethanol and organic binder system (B90-S2075) and homogenised in a tubular mixer. B90 is an organic binder mixture to a plasticiser S2075. The binder used in preparing the granules (e.g. PVA, PMMA) should have little or no solubility in the solvent used in preparing the second/diamond slurry (e.g. ethanol).

The granules prepared from the first slurry are then added to the second slurry and homogenised to form a dispersion of granules in the diamond slurry. The amount of the added granules represents around 5-30% of the total mass of solids in the diamond slurry. The diamond slurry with added granules was then tape cast to produce paper with a required thickness. Discs of the prepared paper were cut and introduced into a Nb cup such that a 2.2 mm PCD thickness is achieved after HPHT sintering onto a WC—Co Substrate.

Pre-composites were pressed by pill technology to form a green-body, de-binded at 600° C. in an inert atmosphere to remove organic binders and outgassed above 1000° C. under vacuum.

The green body was then sintered under high pressure high temperature HPHT conditions at 5 GPa and 1400° C. The PCD cutter was recovered, processed and analysed.

The results are discussed below with reference to FIG. 3.

Example 2

In a second example, cBN powder (grade 6) was mixed with PVA organic binder and de-ionised water in accordance with the method described in example 1 above and a second slurry of diamond powder with a mixture of different grain sizes, ranging between around 1 micron to around 10 microns (Grade 6) was prepared using a solvent such as ethanol and organic binder system (B90-S2075) and homogenised in a tubular mixer, also as described above. Green bodies as described above were prepared and sintered and the PCD cutter recovered. The process was repeated to form cutters with cBN additions forming 7.5 wt % and 20 wt % of the PCD composite. The process was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was substantially larger than that of the diamond grains, (around 30-45 microns whereas the average diamond grain size was around 6 microns).

The results are discussed below with reference to FIG. 4.

Example 3

In a third example, cBN powder (grade 9) was mixed with PVA organic binder and de-ionised water in accordance with the method described in example 1 above and a second slurry of diamond powder with a mixture of different grain sizes, ranging between around 9 micron to around 16 microns (Grade 12) was prepared using a solvent such as ethanol and organic binder system (B90-S2075) and homogenised in a tubular mixer, also as described above. Green bodies as described above were prepared and sintered and the PCD cutter recovered. The process was repeated to form cutters with cBN additions forming 7.5 wt % and 20 wt % of the PCD composite. The process was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was substantially equal to that of the diamond grains, (around 12 microns (Grade 12). It was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was larger than that of the diamond grains, (around 30 microns (a mixture of Grades 20 and 40) whereas the average diamond grain size was around 12 microns), and it was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was substantially greater than that of the diamond grains, (around 45 microns (Grade 45) whereas that of the diamond grains was around 12 microns).

The results are discussed below with reference to FIG. 5.

Example 4

In a fourth example, cBN powder (grade 12) was mixed with PVA organic binder and de-ionised water in accordance with the method described in example 1 above and a second slurry of diamond powder with a mixture of different grain sizes, ranging between around 20 micron to around 25 microns (Grade 22) was prepared using a solvent such as ethanol and organic binder system (B90-S2075) and homogenised in a tubular mixer, also as described above. Green bodies as described above were prepared and sintered and the PCD cutter recovered. The process was repeated to form cutters with cBN additions forming 7.5 wt % and 20 wt % of the PCD composite. The process was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was slightly greater than that of the diamond grains (around 30 microns (a mixture of Grades 20 and 40) whereas the average diamond grain size was around 22 microns), and it was also repeated to obtain cutters comprising 7.5 wt % cBN and 20 wt % cBN where the average grain size of the cBN was substantially greater eg around double than that of the diamond grains, (around 45 microns (Grade 45) whereas that of the diamond grains was around 22 microns).

The results are discussed below with reference to FIG. 6.

Various samples of PCD material were prepared and analysed by subjecting the samples to a number of tests. The results of these tests are shown in FIGS. 3 to 6.

A number of PCD compacts formed according to Example 1 were compared in a vertical boring mill test with a commercially available polycrystalline diamond cutter element having the same average diamond grain size as that of the two examples tested. The first example tested included around 10 vol % (in this example around 10 wt %) second phase (in this example cBN granules) and the second embodiment tested included around 30 vol % (in this example around 30 wt %) second phase (in this example cBN granules). In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically in FIG. 3. The results provide an indication of the total wear scar area plotted against cutting length. It will be seen that the PCD compacts formed according to example 1 (as denoted by reference lines 100, 102) were able to achieve a significantly greater cutting length than that occurring in the conventional PCD compact (as denoted by reference line 104) which was subjected to the same test for comparison. Furthermore, in the example formed of around 10 wt % cBN as the second phase (102), a smaller wear scar area than the conventional PCD compact (104) in this test was achieved.

A number of PCD compacts formed according to Example 2 were compared in a vertical boring mill test with a commercially available polycrystalline diamond cutter element having the same average diamond grain size as that of the examples tested. The first version tested included around 7.5 wt % second phase (in this example Grade 6 cBN granules), the second version tested included around 20 wt % of Grade 6 cBN granules, the third version tested included around 7.5 wt % Grade 45 cBN granules and the fourth version tested included around 20 wt % Grade 45 cBN granules. In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically in FIG. 4. It will be seen that the PCD compacts formed according to Example 2, (as denoted by reference lines 200,202,203,204) gave end wear scars (200′,202′,203′,204′). The conventional PCD compact (as denoted by reference line 205 and wear scar image 205′) was subjected to the same test for comparison. In the version formed of cBN as the second phase in which the average grain size of the cBN granules was approximately double that of the average grain size of the diamond grains (203), a smaller wear scar area than the conventional PCD compact (104) in this test was achieved where the average grain size of the diamonds was fine (around 6 microns).

A number of PCD compacts formed according to Example 3 were compared in a vertical boring mill test with a commercially available polycrystalline diamond cutter element having the same average diamond grain size as that of the examples tested. The first version tested included around 7.5 wt % second phase (in this example Grade 9 cBN granules), the second version tested included around 20 wt % of Grade 9 cBN granules, the third version tested included around 7.5 wt % Grade 12 cBN granules and the fourth version tested included around 20 wt % Grade 12 cBN granules. The fifth version tested included around 7.5 wt % of a mixture of Grades 20 and 40 cBN granules the sixth version tested included around 20 wt % of a mixture of Grades 20 and 40 cBN granules, the seventh version tested included around 7.5 wt % of Grade 45 cBN granules, and the eighth version tested included around 20 wt % of Grade 45 cBN granules.

In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically in FIG. 5. It will be seen that the PCD compacts formed according to Example 3, (as denoted by reference lines 300,302,303,304,305,307,308,309) gave end wear scars denoted by images 300′, 302′, 303′, 304′, 305′, 307′, 308′, and 309′ respectively. The conventional PCD compact (as denoted by reference line 306 and wear scar image 306′) was subjected to the same test for comparison. In the versions formed of cBN as the second phase in which the average grain size of the cBN granules was approximately double or greater than that of the average grain size of the diamond grains, a smaller wear scar area (307-309′) than the conventional PCD compact (306) in this test was achieved where the average grain size of the diamonds was relatively fine (around 12 microns).

A number of PCD compacts formed according to Example 4 were compared in a vertical boring mill test with a commercially available polycrystalline diamond cutter element having the same average diamond grain size as that of the examples tested (Grade 22 having an average grain sioze of around 22 microns which may be considered to be coarse grained). The first version tested included around 7.5 wt % second phase (in this example Grade 12 cBN granules), the second version tested included around 20 wt % of Grade 12 cBN granules, the third version tested included around 7.5 wt % of a mixture of Grades 20 and 40 cBN granules and the fourth version tested included around 20 wt % of a mixture of Grades 20 and 40 cBN granules. The fifth version included around 20 wt % Grade 45cBN granules. In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically in FIG. 6. It will be seen that the PCD compacts formed according to Example 4, (as denoted by reference lines 400,402,404,408,410) gave end wear scars (400′,402′,404′,408′,410′). The conventional PCD compact (as denoted by reference line 406 and wear scar image 406′) was subjected to the same test for comparison. In the version formed of cBN as the second phase in which the average grain size of the cBN granules was approximately double that of the average grain size of the diamond grains (410), a smaller wear scar area than the conventional PCD compact (406) in this test was achieved where the grain size of the diamonds was coarse (around 22 microns).

Whilst not wishing to be bound by a particular theory, it is believed that the fracture performance of PCD may be improved through the introduction of a second phase comprising particles or grains that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % or wt % of the body of polycrystalline superhard material in a PCD matrix according to some embodiments described herein. The second phase particles of this nature are believed to promote crack bifurcation or multiple crack fronts in the PCD material in use, resulting in a redistribution of available strain energy or energy release rate (G) amongst the various crack tips, and/or favourably divert cracks in the PCD material. A material that is able to generate multiple cracks under loading would behave tougher than a material with only one major crack since multiple crack fronts ensures that the net energy supplied to the material is divided between several cracks, resulting in a much slower rate of crack growth through the material. The end result in application of the PCD material including such micro-defects is that, in use, the number of cracks initiated on the wear scar may be increased as compared to conventional PCD, thus reducing the strain energy available for each individual crack, hence slowing the growth rate, and the generation of shorter cracks. The ideal case is where the wear rate is comparable to the crack growth rate, in which case no cracks will be visible behind the wear scar thereby forming a smooth wear scar appearance with no chips or grains pulled out of the sintered PCD.

The addition of such a second phase may also have the effect of increasing the thermal stability of the PCD through the resultant lower cobalt content in the material of the invention compared to conventional PCD.

The size, shape and distribution of these second phase particles, grains, granules or agglomerates may be tailored to the final application of the PCD material. It is believed possible to improve fracture resistance without significantly compromising the overall abrasion resistance of the material, which is desirable for PCD cutting tools.

Thus, it is believed that embodiments may provide a means of toughening PCD material without compromising its high abrasion resistance. The concept works by enabling the creation of multiple crack-fronts or defects which help to redistribute or dissipate the available fracture energy. These defects may also promote crack bifurcations, which is another energy dissipation mechanism. The end result is that there is insufficient energy available to each individual crack to enable it to propagate quickly and hence this may significantly slow down the rate of crack growth.

The vertical borer test results of these engineered structures show a considerable increase in PCD cutting tool life compared to conventional PCD, and with no degradation in abrasion resistance.

Observation of the wear scar development during testing showed the material's ability to generate large wear scars without exhibiting brittle-type micro-fractures (e.g. spalling or chipping), leading to a longer tool life

Thus, examples of a PCD material may be formed having that a combination of high abrasion and fracture performance.

The PCD element 10 described with reference to FIG. 1 may be further processed after sintering. For example, catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be done by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be provided which may further enhance the thermal stability of the PCD element.

Furthermore, the PCD body in the structure of FIG. 1 comprising a PCD structure bonded to a cemented carbide support body may be created or finished by, for example, grinding, to provide a PCD element which is substantially cylindrical and having a substantially planar working surface, or a generally domed, pointed, rounded conical or frusto-conical working surface. The PCD element may be suitable for use in, for example, a rotary shear (or drag) bit for boring into the earth, for a percussion drill bit or for a pick for mining or asphalt degradation.

While various examples have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed. For example, the second phase may be introduced into the PCD in various ways. The dispersed second phase particles may be localized inside the binder pools or in between diamond grains, depending on the sizes. 

What is claimed:
 1. A superhard polycrystalline construction comprising: a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising: a superhard phase, and a second phase dispersed in the superhard phase, the superhard phase comprising a plurality of inter-bonded superhard grains; wherein the second phase comprises particles or grains that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % or around 1 to around 30 wt % of the body of polycrystalline superhard material.
 2. A superhard polycrystalline construction according to claim 1, wherein the superhard grains comprise natural and/or synthetic diamond grains, the superhard polycrystalline construction forming a polycrystalline diamond construction.
 3. A superhard polycrystalline construction according to claim 1, further comprising a non-superhard phase comprising a binder phase.
 4. A superhard polycrystalline construction according to claim 3, wherein the binder phase comprises cobalt, and/or one or more other iron group elements, such as iron or nickel, or an alloy thereof, and/or one or more carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table.
 5. A superhard polycrystalline construction according to claim 3, wherein the particles or grains of the second phase are such that they do not dissolve in the binder phase material and thereby remain unsintered in the body of polycrystalline material and form defects in the polycrystalline material.
 6. A superhard polycrystalline construction according to claim 1, wherein the second phase particles or grains comprise any one or more of an oxide material, a carbide material, alumina, zirconia, yttria, silica, tantalum oxide, cBN, PCBN, hexagonal boron nitride, tungsten carbide, hafnium carbide, zirconium carbide, silicon carbide, or any combination thereof.
 7. A superhard polycrystalline construction according to claim 1, wherein the second phase particles or grains have a grain size substantially equal to or between around 1.5 to around 3 times the average grain size of the superhard grains.
 8. A superhard polycrystalline construction according to claim 1, wherein the second phase particles or grains form between around 1 to around 30 volume % of the body of polycrystalline superhard material.
 9. A superhard polycrystalline construction according to claim 1, wherein the second phase particles or grains form between around 5 to around 15 volume % of the body of polycrystalline superhard material.
 10. A superhard polycrystalline construction according to claim 1, wherein at least a portion of the body of superhard material is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.
 11. A superhard polycrystalline construction as claimed in claim 13, wherein the thermally stable region comprises at most 2 weight percent of catalyst material for diamond.
 12. A superhard polycrystalline construction according to claim 1, wherein the second phase particles or granules have a different stiffness and/or fracture toughness than that of the superhard phase.
 13. A method of forming a superhard polycrystalline construction, comprising: providing a mass of particles or grains of superhard material; providing a mass of second phase grains or particles comprising particles or grains of a material that do not chemically react with the superhard grains, and/or do not inter-grow, and form between around 1 to 30 volume % of the body of polycrystalline superhard material; combining the mass of superhard material and the mass of second phase grains to form a pre-sinter assembly; and treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 5 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the second phase being dispersed in the polycrystalline material and forming between around 1 to around 30 vol % of the body of polycrystalline superhard material, any residual catalyst/solvent at least partially filling a plurality of the interstitial regions.
 14. A method according to claim 13, wherein the step of providing a mass of grains of superhard material comprises providing a mass of diamond grains.
 15. A method according to claim 14, wherein the step of providing a mass of diamond grains comprises providing a mass of grains having a first fraction having a first average size and a second fraction having a second average size, the first fraction having an average grain size ranging from about 10 to 60 microns, and the second fraction having an average grain size less than the size of the coarse fraction.
 16. The method of claim 15, wherein the second fraction has an average grain size between around 1/10 to 6/10 of the size of the first fraction.
 17. The method of claim 13, wherein the average grain size of the first fraction is between around 10 to 60 microns, and the average grain size of the second fraction is between about 0.1 to 20 microns.
 18. The method of claim 15, wherein the weight ratio of the first fraction to the second fraction ranges from about 50% to about 97%, the weight ratio of the second fraction ranging from about 3% to about 50 weight %.
 19. The method of claim 15, wherein the step of providing a mass of grains of superhard material comprises providing three or more grain size modes to form a multimodal mass of grains comprising a blend of grain sizes having associated average grain sizes.
 20. The method of claim 13, wherein the step of providing a mass of second phase grains or particles comprises providing second phase particles or grains comprising any one or more of an oxide material, a carbide material, alumina, zirconia, yttria, silica, tantalum oxide, cBN, PCBN, hexagonal boron nitride, tungsten carbide, hafnium carbide, zirconium carbide, silicon carbide, or any combination thereof.
 21. The method of claim 13, wherein the step of combining the masses of particles or grains comprises coating the superhard grains or particles with the particles or grains of second phase material which does not chemically react with the particles or grains of superhard material. 